1. Field of The Invention
The present invention relates generally to an active suspension system for a vehicle which is operable to suppress bouncing, rolling, or pitching motion of a vehicle body. More particularly, the invention relates to an active suspension system which is responsive to vehicle attitude change to adjust pressure of working fluid cylinders interposed between suspension members and a vehicle body with variations different between increase in the pressure of the working fluid cylinder and decrease in the pressure thereof for providing low friction between a piston rod and a seal ring of the working fluid cylinder.
2. Description of The Background Art
Japanese Patent First Publication No. 63-130418 discloses an active suspension control system for a vehicle. This system includes a vertical acceleration sensor, a lateral acceleration sensor and a longitudinal acceleration sensor, which monitor vehicle attitude change. The system is operable to control pressure control valves to adjust pressure supplied to hydraulic cylinders disposed between road wheels and a vehicle body respectively according to values monitored by the sensors, for suppressing vehicle attitude change.
In such a working fluid cylinder utilized in an active suspension system, highly pressurized working fluid is supplied to a pressure chamber of the cylinder to displace a piston rod for suppressing vibrations of a vehicle body. Thus, if a sealing member is utilized for preventing working fluid in the pressure chamber from leaking, friction created between a piston rod and the sealing member is subject to increase as pressure in the pressure chamber is elevated, causing the piston rod to slide unsmoothly.
In order to avoid such drawbacks, Japanese Patent First Publication No. 64-87902 discloses an improved hydraulic cylinder. This cylinder includes sealing members for sealing a piston which defines working chambers and a check valve arranged between a reservoir chamber and a reservoir tank. The sealing member is adapted for allowing hydraulic fluid to leak therethrough. The check valve allows hydraulic fluid to be directed from the reservoir chamber to the reservoir tank. Supplying pressure to the working chambers causes the hydraulic fluid to leak between the sealing members and corresponding parts. The leaked fluid forms a hydraulic film on the parts to reduce friction between the sealing members and the parts, resulting in smooth motion of the piston rod.
If the above prior art hydraulic cylinder is used in an active suspension system for a vehicle, friction is created between a piston rod and a sealing member which is smaller than that of the above mentioned liquid-tight seal type hydraulic cylinder. However, even in such a hydraulic cylinder, a fluid supply to the hydraulic cylinder at the high pressures required for active suspension control causes the sealing member to seize against the piston rod, resulting in increased friction to interfere with smooth sliding motion of the piston rod. Thus, road vibrations through wheels are transmitted directly to a vehicle body, giving passengers an unpleasant feeling.